1. Technical Field of the Invention
The present invention pertains in general to techniques for maximizing utilization of communication capacity in a communication network and, more particularly, to a method and apparatus for performing intra-cell handovers of existing communication connections in a cellular telephone network base station to optimize utilization of communication equipment.
2. Description of Related Art
In a cellular telephone network each base station is outfitted with communication equipment for effectuating communication with mobile stations via a communication channel on an air interface. The communication equipment often has different channel rate capacities and restrictions. To better understand these distinctions the following terms are defined as follows:
Channel Rate--The channel rate indicates the communication rate presently in use for a traffic channel. Possible values are full rate and half rate.
Channel Rate Capacity--Channel rate capacity denotes the communication rate capability of a channel. Possible values are dual rate and full rate only.
Dual Rate Channel Pair--A dual rate channel pair denotes a pair of channels on dual rate communication equipment which together provide a resource capable of carrying one full rate connection or two half rate connections. A dual rate channel pair consists of the channels corresponding to the air interface time slots TS0/TS3, TS1/TS4, or TS2/TS5 in the Personal Digital Cellular (PDC) protocol.
Dual Rate Communication Equipment--Dual rate communication equipment denotes base station communication equipment which provides resources capable of carrying full rate and half rate connections. Dual rate communication equipment provides three dual rate channel pairs.
Dual Rate Traffic Channel--A dual rate traffic channel denotes one of two traffic channels included in a dual rate channel pair. A dual rate traffic channel is capable of carrying a half rate connection. Together with the other dual rate traffic channel in the dual rate channel pair, the two dual rate traffic channels are capable of carrying a full rate connection.
Full Rate Only Communication Equipment--Full rate only communication equipment denotes base station communication equipment providing resources capable of carrying full rate connections. Full rate communication equipment provides three full rate channels.
Full Rate Only Traffic Channel--A full rate only traffic channel denotes a traffic channel capable of carrying full rate connections only.
Full Rate Traffic Channel--A full rate traffic channel represents a traffic channel carrying a full rate connection. In the PDC air interface, a full rate traffic channel corresponds to one full rate time slot, TS0, TS1 and TS2. One full rate time slot corresponds to two half rate time slots.
Half Rate Traffic Channel--A half rate traffic channel represents a traffic channel capable of carrying a half rate connection. In the PDC air interface, a half rate traffic channel corresponds to one half rate time slot, TS0, TS1, TS2, TS3, TS4 or TS5.
Full rate only mobile station--A full rate only mobile station is a mobile station which can only utilize a full rate connection.
Dual rate mobile station--A dual rate mobile station is a mobile station which can utilize a full rate connection or a half rate connection.
Full Rate Connection (FRC)--A full rate connection is a communication connection between a base station and a mobile station carried on either a full rate only traffic channel or a full rate traffic channel.
Half-Rate Connection (HRC)--A half rate connection is a communication connection between a base station and a mobile station carried on a half rate traffic channel.
Half Rate Capable Connection (HRCC)--A half rate capable connection is the equivalent of a full rate connection in that it is a communication connection between a base station and a dual rate mobile station carried on a full rate only traffic channel, however, the underlying communication requirements only necessitate the use of a half rate connection despite the use of a full rate only traffic channel. For example, whereas a facsimile or modem communication between the base station and mobile station requires the use of a full rate connection, a voice communication requires only a half rate connection. Thus, in circumstances where a half rate traffic channel is unavailable, a half rate capable connection is used. In these circumstances, half of the communication capacity of the full rate only traffic channel is unnecessary and essentially lost.
Referring now to FIG. 1, there is illustrated a functional block diagram representing channel rate capacities and restrictions of communication equipment in a cellular telephone base station. Although the description is directed toward a base station implementing Personal Digital Cellular (PDC) protocol, the concepts and issues presented also apply to other protocols. A cellular telephone base station 100 comprises full rate only equipment 110 including, for example, three full rate only traffic channels 130, 140 and 150 on time slots TS0, TS1 and TS2 respectively. The cellular telephone base station 100 further comprises dual rate equipment 120 including, for example, three dual rate channel pairs 160, 170 and 180. Each dual rate channel pair comprises two dual rate traffic channels 161, 162, 171, 172, 181 and 182 on time slots TS0, TS1, TS2, TS3, TS4 and TS5 respectively. Each of the full rate only traffic channels 130, 140 and 150 can carry either a full rate connection or a half rate capable connection. Since the underlying communication requirement of the half rate capable connection is equivalent to a half rate connection, half of the channel communication capacity is lost when carrying a half rate capable connection on a full rate only traffic channel. On the other hand, each dual rate channel pair 160, 170 and 180 can carry either a single full rate connection or two half rate connections. A full rate connection, however, can not be split between two separate dual rate channel pairs.
As communication connections between the cellular telephone base station 100 and mobile stations are established and subsequently terminated, situations result where existing communication connections become allocated to the communication equipment in a less than optimal fashion. The following examples are provided to illustrate how such situations can arise. Other situations are possible and the examples given are not intended to be an exhaustive list.
In a first example, all dual rate traffic channels 161, 162, 171, 172, 181 and 182 of the dual rate equipment 120 are in use as are all the full rate only traffic channels 130, 140 and 150 of the full rate only equipment 110. If two half rate connections on the dual rate traffic channels of two distinct dual rate channel pairs subsequently terminate, for example dual rate traffic channels 161 and 171, no new full rate connections can be established. No new full rate connections can be established due to the restriction that a full rate connection can not be split between distinct dual rate channel pairs even though the cellular telephone base station 100 has available communication capacity (dual rate traffic channels 161 and 171) to carry a new full rate connection.
In a second example, all the full rate only traffic channels 130, 140 and 150 of the full rate only equipment 110 are in use and all but one of the dual rate traffic channels, for example dual rate traffic channel 182, of the dual rate equipment 120 are also in use. If a half rate connection on one of the dual rate traffic channels 161, 162, 171 and 172 of the two respective dual rate channel pairs 160 and 170 terminates, for example dual rate traffic channel 172, a situation arises wherein the cellular telephone base station 100 has available communication capacity (dual rate traffic channels 172 and 182) to carry a full rate connection but since a full rate connection can not exist on distinct dual rate channel pairs, the cellular telephone base station 100 can not carry a new full rate connection.
In a third example, all the dual rate channel traffic channels 161, 162, 171, 172, 181 and 182 are in use. Furthermore, all the full rate only traffic channels 130, 140 and 150 are in use with at least one full rate only traffic channel, for example full rate only traffic channel 150, carrying a half rate capable connection. If a half rate connection on one of the dual rate traffic channels, for example dual rate traffic channel 172 terminates, the cellular telephone base station 100 can not carry another full rate connection even though it has available communication capacity (dual rate traffic channel 172 and the lost communication capacity of the full rate only traffic channel 150) to carry a new full rate connection.
Although other examples exist, these three scenarios illustrate how a less than optimal utilization of the communication equipment of the cellular telephone base station 100 can result when calls are terminated. There is a need therefore, for a method and apparatus to reallocate existing communication connections between communication channels in a cellular telephone base station as existing communication connections are terminated in order that utilization of the communication equipment can be optimized.